This application discloses an invention which is related, generally and in various embodiments, to metal shearing knives or tools and their edges. Metal shearing knives or tools include shearing knives or any tool which separates metal. Metal shearing knives are made in a number of configurations, including but not limited to: straight, with one to four straight shearing edges; rotary, with one or two circular shearing edges; curved, with one to four curvilinear shearing edges; and helical, typically with one compound curved shearing edge.
When shearing low to mid-strength metals, i.e. less than approximately 500 MPa tensile strength, edges of shear knives typically reach end of life when the edge gets dull (worn) and are no longer able to shear the metal and provide the desired sheared edge quality. Dull knife edges typically produce undesirable burr on the sheared edge.
Abrasive wear of the knife edge, as it moves through the metal being sheared, wears the originally sharp edge to a dull edge. When the edges of a knife becomes too dull/worn to produce the desired quality of sheared edge, the shear knife edge typically is resharpened by grinding or machining to remove knife material from one or both of the two surfaces that intersect and form the knife edge.
Examples of prior art knife edges are shown in FIGS. 1-4. FIG. 1 shows a prior art knife edge 100 of a conventional metal shearing knife. Prior art knife edge 100 has a cutting surface 102 disposed between two surfaces 104, 106. In this illustration, the surfaces 104, 106 are portions of a first face/outside diameter 108 and a second face 110. Prior art knife edge 100 has a ground surface finish with surface finish marks 112 substantially parallel with cutting surface 102.
FIG. 2 shows a prior art knife edge 200 of a conventional rotary knife. Prior art knife edge 200 has a cutting surface 202 disposed between two surfaces 204, 206. In this illustration, the surfaces 204, 206 are portions of an outside diameter 208 and a face 210. Surface 204 of outside diameter 208 has surface finish marks 212a substantially parallel with cutting surface 202. Surface 206 of face 210 is lapped with non-directional surface finish marks 212b. 
FIG. 3 shows a prior art knife edge 300 of a conventional straight knife. Prior art knife edge 300 has a cutting surface 302 disposed between two surfaces 304, 306. In this illustration, the surfaces 304, 306 are portions of a first face 308 and a second face 310. The surfaces 304, 306 have a ground surface finish created by a vertical spindle grinder grinding wheel with surface finish marks 312 at a bias to the cutting surface 302.
FIG. 4 shows a prior art knife edge 400 of a conventional rotary knife. Prior art knife edge 400 has a radiused cutting surface 402 disposed between two surfaces 404, 406. In this illustration, the surfaces 404, 406 are portions of outside diameter 408 and face 410. Surfaces 404, 406 blend with radiused cutting surface 402, and a single-point machined surface finish with surface finish marks 412 substantially parallel with cutting surface 402 on surfaces 404, 406 of the outside diameter 408 and face 410, and on cutting surface 402.
Abrasive wear rates of shear knife edges can be reduced by selection of knife metallurgy (material composition, heat treatment, and hardness) to produce harder, more wear-resistant knife edges that slow the rate of wear and prolong edge life.
In the 1960's, researchers began developing higher strength steels for military and specialty applications. Since that time, higher and higher strength steels have been developed and produced. HSLA steels (High Strength Low Alloy) and AHSS (Advanced High Strength Steels) are commonly used to achieve lower weight, higher strength, and lower cost products, such as automobiles, pipe, tube, structures, and fabrications. These higher strength steels have tensile strengths well above 500 MPa, to as high as 1600 MPa in current production, and to over 2000 MPa under development.
A problem associated with shearing these higher strength steels is the early failure of shear knife edges by fracturing. Higher strength steels, with 2 times to 4 times the tensile strengths of low to mid-strength steels, cause edges of conventional knives to fail by pieces breaking out of the knife edges at rates 5 to 10 times greater than dulling by abrasive wear.
The accelerated failure of the shear knife edges is due to high contact stresses in the knife edges resulting from the higher forces required to shear the high strength steel. The higher stresses in the knife edges cause fatigue fractures to start earlier and to propagate faster, until edge spalling occurs. The higher the tensile strength and thickness of the metal being sheared, the higher the stress in the knife edges and the faster the edges fail. ‘Fatigue life’ related to metal shearing knives is a relative property of a knife edge compared with another knife edge, when the edges are run under similar conditions, shearing the same or similar material (grade, strength, and thickness). If a knife edge fails due to fatigue cracks developing and propagating until a piece of the knife edge breaks out of the knife, producing an unacceptable condition of the sheared edge of the material being sheared, the fatigue life of that knife edge is considered to be lower than a knife edge shearing the same material without failing in this manner. When comparing the fatigue life of different objects, typically the length of a run until failure is measured. Different steel mills record length of run in different units, such as: million feet (MFT) of lineal coil length; kilometers (km) of lineal coil length; number of coils, tons, number of work shifts, hours.
Attempts to reduce the rate of fatigue failure, using knives made from tougher grades of tool steel, have had mixed success, ranging from 20% to 30% longer life to equal or sometimes lower life. These tests do not indicate significant, repeatable increase of fatigue life by using this approach.